1. Field of the Invention
This invention relates to video data compression.
2. Description of the Prior Art
Some video data compression systems, such as systems defined by the MPEG II standard.sup.1, use a number of different coding techniques to encode successive pictures of a video signal. FNT .sup.1 Motion Picture Experts Group II standard, described in the ISO/IEC Publication DIS 13818/2, "Information Technology--generic coding of motion pictures and associated audio information", March 1995
Typically, the video signal is divided into successive groups of pictures (GOPs). Within each GOP at least one picture is encoded as an "I-picture", or intra-picture, using only information present in that picture itself. This means that I-pictures can later be decoded without requiring information from other pictures, and so provide random entry points into the video sequence. However, the converse of this is that the encoding of I-pictures cannot make use of the similarity between successive pictures, and so the degree of data compression obtained with I-pictures is only moderate.
Further pictures within each GOP may be encoded as "P-pictures" or predicted pictures. P-pictures are encoded with respect to the nearest previous I-picture or P-picture, so that only the differences between a P-picture and the previous P- or I-picture needs to be transmitted. Also, motion compensation is used to encode the differences, so a much higher degree of compression is obtained than with I-pictures.
Finally, some of the pictures within a GOP may be encoded as "B-pictures" or bidirectional pictures. These are encoded with respect to two other pictures, namely the nearest previous I- or P-picture and the nearest following I- or P-picture. B-pictures are not used as references for encoding other pictures, so a still higher degree of compression can be used for B-pictures because any coding errors caused by the high compression will not be propagated to other pictures.
Therefore, in each GOP there are (up to) three classes of picture, I-, P- and B- pictures, which tend to achieve different degrees of compression and so tend to require different shares of the overall available encoded bit stream. Generally, I-pictures require a large share of the available transmission or storage capacity, followed by P-pictures, and followed by B-pictures.
However, this is only a general rule, and while the trends might remain the same, the number of bits required to achieve an equal subjective quality of encoding for the pictures in a GOP may vary from GOP to GOP. Any errors in the allocation of transmission or storage capacity between pictures can lead to uneven or reduced subjective quality of the encoded frames, because errors, artifacts and noise due to compression will not be distributed evenly between the frames of the GOP.
It has been proposed to allocate shares in the available encoded bitstream to individual pictures within a GOP on the basis of the degree of compression obtained for corresponding pictures in a preceding GOP.
It is a constant aim in the field of video compression to improve the subjective quality of the encoded (and subsequently decoded) images.